The present invention relates to a magnetic ring for detecting the rotation of an object and in particular to such a magnetic ring making use of synthetic resin material in which magnetic particles are dispersed.
It is often necessary to electrically detect the rotation or rotational speed of an object. For instance, it is often desired to measure the rotational speed of a shaft in industrial applications, and it is often desirable to measure the rotational speed of a wheel in automotive applications. For instance, in evaluating the speed of an automobile, one can obtain a highly accurate speed reading by measuring the rotational speed of a wheel itself; and it is absolutely necessary to measure the rotational speed of a wheel in anti-skid control of a brake system.
In such applications, it has been common to use a toothed gear which is mounted on a rotating object and to measure its rotational speed by counting pulses generated from a magnetic induction device placed adjacent to the toothed gear. Since such a toothed gear is typically press fitted into a bore or onto a shaft, it must be capable of withstanding the stress arising from such a press fitting, and it is typically made of a metal such as steel. Since hobbing of a gear which is a fairly time consuming and expensive process, is necessary and a substantial dimensional precision is necessary for satisfactory press fitting, such a metallic ring for detecting rotational speed tends to be costly.
Magnetic material consisting of synthetic resin in which ferromagnetic powder is dispersed is known and it can be formed into a ring having a plurality of gear teeth or magnetized to have alternating magnetic poles along its circumference at very low cost. Furthermore, it is substantially lighter in weight than a conventional toothed gear ring made of steel. However, such material lacks the mechanical strength required for press fitting. Such material, particularly in the, form of a toothed gear, is prone to damage during assembly and in use, and its reliability and durability are insufficient for practical purpose.